Conventionally, an LED array panel such as is used in an optical printer head or the like has been produced by arranging a plurality of pellets of LEDs (light-emitting diodes) in one line and a plurality of pellets of driving devices in another line both on a long and narrow substrate, as disclosed in Japanese Laid-Open Patent Application No. S60-72281. This is because, whereas it is impossible to produce a pellet that is longer than about 10 mm, an optical printer head is usually required to be longer than 200 mm. In addition, to obtain satisfactorily high print quality, the LED pellets need to be arranged in a straight line, and therefore it is customary to make the substrate of ceramic or other material that is resistant to deformation.
However, an optical printer that can print on a large sheet of paper such as an A0-size sheet requires a substrate that is about 1,100 mm long, and, if this substrate is made of ceramic, it will be unduly heavy. Using such a heavy substrate necessitates accordingly large supporting components, and thus ends in making the printer as a whole unduly large.
To overcome this problem, a method has been devised in which LED pellets are arranged on a light-weight substrate made of resin. FIGS. 1(a) and 1(b) show the structure of an LED array panel produced by such a method. FIG. 1(a) is a plan view of an LED array panel that uses a substrate made of resin, and FIG. 1(b) is a sectional view taken along line A--A of FIG. 1(a). The long and narrow substrate 1 of resin is a printed circuit board 1,100 mm long, 35 mm wide, and 0.6-1.5 mm thick. Its base 11 is made of glass-epoxy prepreg, a mixture of glass-epoxy prepreg and ceramic, paper-epoxy prepreg, or a similar material. The substrate 1 has printed patterns 12a to 12d that are formed on its base 11 by coppering, printing, metalizing, or a similar method. These patterns are preferably formed over as large an area as possible to increase the strength of the substrate 1, to ease the dissipation of heat from the LEDs formed in the LED pellets 2, and to reduce the effect of wiring resistance on the voltage of the electric power supplied to the LEDs. In fact, in this example, the printed patterns 12a and 12b, which are for the negative side (ground side) and the positive side, respectively, of the supplied power, are formed over as large an area as possible, and, in addition, the printed pattern 12d, which is for heat dissipation, is formed on the bottom surface of the substrate 1 as well. If necessary, a multilayer-wiring printed circuit board may be used as the substrate 1. The printed pattern 12c is for the connection of drivers (described later) for driving the LEDs.
Along the length of the substrate 1, pellets 2 of LEDs are arranged in a line, and pellets 3 of drivers for driving the LEDs are arranged in another line parallel thereto. These pellets are, with thermosetting adhesive 4 and 5, fixed to the substrate 1 on one side (the upper side in FIG. 1(a)) thereof with respect to the width center thereof. In this example, the dimension C in the figure is about 10 mm. One-sided placing of the pellets helps to reduce the width of the printer head. Each of the LED pellets 2 is formed as a so-called monolithic array; for example, it is 8 mm long and 0.6 mm wide, and has, on its top surface and along its length, a linear array of light-emitting regions 21 that provide a resolution of 500 dpi (dots per inch), with each light-emitting region provided with an individual electrode 22; in addition, it also has, on its bottom surface, a common electrode (not shown). A plurality of such LED pellets 2 are so arranged that their light-emitting regions 21 form a straight line that extends almost over the entire length of the substrate 1; these pellets are fixed to and thereby grounded to the substrate 1 with conducting thermosetting adhesive 4 such as silver paste that contains epoxy resin as its main ingredient.
On the other hand, the driver pellets 3, each having an integrated circuit built inside near its top surface, are arranged in a line approximately parallel to the LED pellets 2, and are fixed to the substrate 1 with non-conducting thermosetting adhesive 5 such as epoxy-resin-based adhesive. Each driver pellet 3 has one end connected to the individual electrode 22 of the corresponding LED pellet 2 with a fine wire-bonding wire 6, and has the other end connected to the printed pattern 12c for the connection of the driver pellets or to the printed pattern 12b for the positive side of the supplied power similarly with a fine wire-bonding wire 6.
The substrate 1 has through holes 13a and cuts 13b that allow it to be fixed to a head supporting member or heat sink.
However, actual production of an LED array panel 10 having a structure as described above proved that it has the following disadvantages.
FIG. 4 shows the results of an experiment in which 113 LED pellets 2 were arranged on a resin substrate 1 with thermosetting adhesive 4 by the use of an automatic bonding machine and then the adhesive was hardened with heat. In FIG. 4, the abscissa represents the position of the individual LED pellets 2, and the ordinate represents the deviation of the individual LED pellets 2 in the direction of the width of the substrate 1 from the line through the LED pellets 2 at both ends. In FIG. 4, the graphs (A) show the deviation observed when the substrate has just been removed from the automatic bonding machine, and these graphs are curved about 0.15 mm downward at their middle, assuming that the LED pellets 2 are arranged on the "upper" side of the substrate 1 with respect to the width center thereof. The graphs (B) show the deviation observed after the heat-hardening of the thermosetting adhesive 4, and these graphs are curved about 0.15 mm upward at their middle. The graphs (C) show the difference between the deviation before the heat-hardening and the deviation after the heat-hardening, and these graphs indicate that the heat-hardening of the thermosetting adhesive 4 results in shifting the curves a maximum of 0.25 mm upward at their middle.
The warping of the array of the LED pellets 2 before the heat-hardening can be reduced by adjusting the automatic bonding machine. The results of an experiment that was performed with the automatic bonding machine appropriately adjusted are shown in FIG. 3. In FIG. 3, as in FIG. 4, the abscissa represents the position of the individual LED pellets 2, and the ordinate represents the deviation of the individual LED pellets 2 in the direction of the width of the substrate 1 from the line through the LED pellets 2 at both ends. The graph (A) shows the deviation observed when the LED pellets 2 are arranged on the substrate 1 after the adjustment of the straightness of the automatic bonding machine, and this graph indicates that the deviation then falls within a range from -0.07 mm to +0.03 mm. However, after the heat-hardening, as the graph (B) shows, the deviation describes a curved line that. is curved about 0.2 mm upward at its middle. This change resulting from the heat-hardening is not due to the movement of the LED pellets 2 during the heat-hardening, but due to the fact that, as shown in FIG. 2, the substrate 1, which has a shape as indicated by broken lines before the heat-hardening, warps upward within its plane as indicated by solid lines after the heat-hardening. This warping results from the LED and driver pellets 2 and 3 being placed on one side (the upper side in FIG. 1(a)) of the substrate 1 with respect to the width center thereof, and it has been found that such warping cannot be avoided as long as the substrate is made of resin.
An object of the present invention is to provide an LED array panel in which LED pellets are arranged accurately in a straight line on a substrate made of resin.
Another object of the present invention is to provide a method of manufacturing an LED array panel in which LED pellets are arranged accurately in a straight line on a substrate made of resin.